“No one wants it in their vineyard, it is quite easily controlled and there is no excuse for having it,” Bell went on to say. “Yet this last year saw some profound powdery mildew in Gisborne, in a year we considered one of our best vintages.”
It seems appropriate therefore to go to a world expert on this insidious disease to work out how best to deal with it. That expert is Peter Margarey an Australia Research Plant Pathologist who was a guest speaker at the 2011 NZW Grape Days.
He describes powdery mildews as being unique in terms of vineyard diseases.
“It doesn’t require moisture, it grows on the outside of the leaf surface and it is a green disease, requiring green tissue to thrive on.”
Consequently any sign of the disease on foliage or fruit that is removed from the vine, cannot survive – unlike a disease like botrytis, which can winter over in dead cell material.
What’s more important, is the disease is driven by the amount of inoculum inherited from last season. Therefore if you can manage the inoculum this season, you are part way there to preventing an outbreak next year.
“Early season control is critical. To win the battle, we need to control next season, this season.”
While it doesn’t require moisture to develop, powdery mildew loves mild temperatures of around 20 to 28°C, with lots of cloud. Its one great hate is ultra violet light.
It can be difficult to determine powdery mildew at first glance, Magarey said, given it is quite similar to many others found within vines. He suggested carrying an eyeglass when checking among the vines, and highlighted a phone ap, which can help the identification process for those out in the field.
(the site is growcare.com.au.)
If there was one point he was adamant in getting across, it was detecting the disease early on, at bud burst. Leave it any longer and it is harder to control.
“You need to monitor the vineyard, get out there and look. Know your target, what you are looking for and look at as many leaves as you can. Do this early in the season so you can detect it early, and keep records so you have a history of where the previous infection areas have been.”
The first signs of powdery mildew will show up in emerging shoots, as the inoculum is carried over within infected buds. (Not ON infected buds.) These shoots, known as flagshoots will show signs of stunting with leaves curled over.
“If you find a flagshoot, you can break it off and throw it on the ground, as the spores can’t survive on dead tissue. But tag the site so you can come back and check if any more flagshoots have appeared.”
Left unmanaged, the spores from the flagshoot will spread incrementally into nearby green tissue, such as other shoots, leaves and berries, with the infection period being only about five to six days.
“Young buds are most susceptible to infection two to three weeks after budburst.”
While the fungal spores are spread by wind, they do not travel great distances with Magarey stating “your vineyard is not affected by your neighbour’s.”
Opening the canopy to more ultra violet light, by pruning, leaf plucking and shoot thinning – are all useful for many other vineyard diseases, but he said they do not make a massive difference with powdery mildew. The most important thing to do is, reduce inoculum reservoirs.
“If you have powdery mildew this year, most or all of the inoculum will have come from within a 200 – 300 metre zone of where the disease was not controlled last year.”
Which brought Magarey to the all important issue of how do you control it? In Australia he said, it was common practice to focus spraying on either side of flowering.
“That’s alright, as it protects the fruit this season, but it doesn’t do much for next year. The result is you need to do it all over again next year and the year after.”
Given the infection starts with the buds turning into a flagshoot, which then spreads to the leaves, and in turn to the green berries – the best way to prevent severity later in the season, is to kill the inoculum at or near budburst.
“This will prevent or reduce the leaf inoculum and the fruit infection in this year coming.”
It also means sprays are more effective, because at the time of control, the inoculum loads are at their lowest level.
Magarey then went on to describe how to take control during the lag phase.
“Mathematically the lag phase is when you get something that is building up slowly and slowly and then gradually it builds up until you get enough of something to really take off. Powdery mildew is a hard disease to catch if it is running away from you, so we want to stop it in the lag phase.”
There is no such thing as an “outbreak” of powdery mildew he said. Instead it is just a case of the inoculum being there and growing incrementally, until the disease is full blown.
Roughly 40 days from bud burst, (give or take depending on your own individual conditions) the disease incidence increases. In Australian conditions Magarey said, 80 days after bud burst severity increases and then fruit infection begins.
“The notion is, if we spray the disease early in the lag phase, just after the spores have started growing in the vineyard, you can stop leaf and fruit infection this year and do very significant things in controlling the disease long term.”
In summary – the disease begins inside an infected bud, carried over from last season. If left it will develop into a leafshoot with spores that can be spread by wind. Those spores will continue to multiply slowly but surely in the right weather conditions of warm temperatures and cloudy skies. The spores will transfer to other shoots, onto leaves and into fruit – reaching severe proportions.
At the end of the season, the disease if not controlled will remain on any green tissue, which is the buds on the old canes not removed. This inoculum will winter over, waiting for more green tissue to develop next season.
However if you take control immediately after budburst, you can prevent any of that continual cycle occurring.
Magarey said controlling that cycle is as easy as spraying thoroughly a couple of times early season, three times with two week intervals – basically at week two, four and six after budburst. ν
